1. Field of the Invention
This invention relates to an outboard motor steering system.
2. Description of the Related Art
Almost all outboard motor steering systems have up to now been of types operated by human power, such as the tiller handle type used to turn the rudder by manually operating the tiller handle attached to the outboard motor and the remote control type used to remotely operate a steering mechanism through a push-pull cable in response to rotation of a steering wheel manipulated by the operator.
Since human-powered steering systems are disadvantageous because they tend to have an unpleasant steering “feel” owing to, for instance, heavy steering load, as taught in Japanese Laid-Open Patent Application Sho 62 (1987)-125996, an add-on mechanism constituted as a separate unit from the outboard motor and used to power-assist the turning of the steering wheel is known. This mechanism typically includes a steering actuator such as a hydraulic cylinder placed on the boat to power-assist the steering through a link mechanism.
Also, as taught in Japanese Laid-Open Patent Application Sho 62 (1987)-166193 ('193), another add-on mechanism similarly constituted as a separate unit from the outboard motor and used to power-assist the turning of the steering wheel is known. This mechanism includes a hydraulic cylinder placed on the boat and is connected to the swivel shaft through an arm. In the system mentioned in '193, a rotation angle sensor is installed at the boat to detect the angle of arm rotation, and the hydraulic cylinder is operated to decrease an error between the angle of steering wheel rotation inputted by the operator and the detected angle of arm rotation such that the boat is steered as desired by the operator.
The add-on steering system using such an actuator also has disadvantages, most notably that its structure is complicated, that it adds to the number and weight of the components, and that it takes up space around the boat.
Attempts have been made to overcome these drawbacks. Japanese Laid-Open Patent Application No. Hei 2(1990)-279495 ('495), for example, teaches a steering system including a steering actuator that is not attached to the boat, but directly attached to the outboard motor, thereby minimizing increase in the number and weight of the constituent components and saving space.
Aside from the above, the add-on steering system using a hydraulic cylinder, as taught in Japanese Laid-Open Patent Application No. Hei 6 (1994)-127475 ('475), typically includes a hydraulic pump, an electric motor for driving the pump and a switch valve installed in a hydraulic pressure circuit for switching the direction of oil flow, etc.
However, the steering system taught by '193 is disadvantageous from the aspect of saving space around the boat, since it needs the rotation angle sensor to be installed at the boat to detect the angle of rotation of the arm that connects the hydraulic cylinder to the swivel shaft.
Further, the steering system taught by '495 is disadvantageous from the aspect of saving space around the outboard motor because in some operating states of the hydraulic cylinder, the hydraulic cylinder projects from the outboard motor in the horizontal direction. This is serious in particular when two outboard motors are installed side by side in a dual motor configuration, the installation space must be enlarged by the amount of projection of the actuator so as to prevent interference between the outboard motors.
Further, the steering system taught by '193 is disadvantageous from the aspect of steering feel, since it needs the angle of arm rotation to be detected, if the arm is distorted or deformed. The sensor value does not detect the angle of rotation of the swivel shaft accurately and hence, it becomes difficult to operate the hydraulic cylinder to achieve steering as desired.
Further, the steering system taught by '475 is also disadvantageous from the aspect of steering feel, since, when the operator operates the electric motor to drive the hydraulic pump with the intention of turning the boat in an opposite direction, the boat can not change the direction due to inertia force exerting thereon. For this reason, in particular when the boat is intended to turn quickly, hydraulic pressure rises sharply and generates a large reaction force that will act on the electric motor. Since this reaction force is transmitted to the hydraulic cylinder and other parts as an impact, it occasionally becomes difficult to turn the boat smoothly, thereby degrading the steering feel.